In an SPM or AFM, prior to scanning the sample surface, usually two engagement steps take place. In a first or coarse engagement step, a cantilever-type probe is moved from a few millimeters (or even centimeters) above the sample surface to a position less than 1 millimeter (for instance a few 100 μm) above. After this step, a fine engagement is started: The cantilever is moved in steps of a few micrometers until it reaches its final starting position for the measurement.
However, in order to save time, it is desired that the first coarse engagement step is done quickly, but without moving the cantilever too fast and too close to the sample and thus damaging the cantilever tip and/or the sample.
Thus, in scanning probe microscope systems, it is desired to situate the probe and the sample in close vicinity prior to the final engagement procedure. This engagement is typically a slow process, which duration scales linearly with the gap that needs to be covered, thus minimizing the duration of this process and the size of this initial gap is desirable to improve throughput of the instrument. While moving the cantilever towards the sample or vice versa, caution should be taken since accidental contact of the cantilever with the sample can damage the tip and/or sample and/or instrument. In particular in case of very polished or transparent samples, it can be difficult to judge the sample height. Furthermore, if the samples that the instrument can measure are large in the vertical dimension, the distance covered by the stage controlling the cantilever-sample separation needs to be large as well. In this case, for low height samples the coarse approach time can be long if the feedback of the position is not reliable and the operator may err on the side of caution.
If the samples that the instrument admits are also large in the horizontal dimension and have significant variations in height, the operator should also take care not to crash against high features when translating laterally.
In this context, the operator has either to:                monitor in-situ the coarse approach by eye. This requires a good view of the sample/measuring head, which usually involves opening any enclosure around the system. The system may be destabilized (temperature, atmosphere, pressure, contamination) by this operation.        monitor the approach by comparing the top-down view focus of sample and probe (typically with microscope optics). This can lead to crashes when the top surface is not easy to focus, for example if the material is transparent or very polished and the surface is clean of contamination. These types of surfaces (very clean, optically flat) are common in SPM. This is also problematic when the sample has a degree of tilt that makes some areas of the sample exceed the depth of field of the microscope optics so not all the field of view can be focused at once.        monitor the approach from a position aside and above sample and probe, thus providing a steep oblique view on the area of interest.        